This patent application from Google, made public last week, goes on to describe alternatives to tabs, and varying ways to make different categories more prominent on a page, possibly based upon how relevant the search engine feels they may be to the query.

Update: Gary Price from SEW pointed out to me that the patent app was filed for less than a month after danny wrote his article. He also tells me to tell you guys that, "many companies have been doing work in what's called federated search for years. One interface to tap many databases." Some more related information at ResourceShelf.